1. Field of the Invention
The present invention relates to a wide angle lens having a long back focus substantially equal to the rear focal length and more particularly to such a wide angle lens useful for taking a picture underwater.
2. Related Background Art
An underwater camera provided with a negative- positive-negative symmetric type wide angle lens is known in the art which has an angle view of about 80.degree. and a F number of about 2.8. However, the known underwater camera has an important drawback that the back focus of the wide angle lens is short for the rear focal length f of the total system. The back focus is only 0.55.f which is too short to provide a sufficient space enough to receive a light measuring device between the lens and the image plane.
One of the well-known methods for extending the back focus is to use a so-called retro-focus type (inverted telephototype) lens. As an example, in the wide angle lens disclosed in Japanese Patent Application laid open No. 13210/1984 the applicant of which is the same as the applicant of the present application, the back focus has successfully been extented to a value of 2.0 f or more according to the prior art method. However, this prior method for the extension of back focus involves some problems.
If the back focus is extented too much, a substantial increase of the total size of the apparatus is needed, which reduces the portability of the underwater camera. In addition, it may cause the problem of water leakage in the underwater camera. On the other hand, if the retro-focus type wide angle lens is designed to have a back focus of about 1.0 f, the exit angle of light is increased too much due to the exit pupil being close to the image side. The increase of the exit angle enhances the field curvature of the lens. In this case, therefore, it is very difficult to obtain good image-forming characteristics of the lens.